Normally, they adapted Tengwar to their new language, taking advantage of their inherent flexibility to other sounds, imitating maybe the way the Falmari of Eldamar represented the Telerin sounds that didn't exist in Quenya. For some obscure reasons we do not know, they found the most usual tehta-mode inappropriate for Sindarin, and they took therefore also example of the original Fëanorian Quanta Sarmë, using distinct tengwar for vowels.

Of the Quessetéma, Wilya was used for a final -u (or -w). There is also an a-tengwa called Osse, not corresponding with any of the older ones known from Quenya; it could have been imported from a Quenya semivowel unknown to us, which belonged to the Quanta-Sarme.

The carriers took vowel values too: The telco was used for i, while the long carrier could have been used for the semiconsonantal initial sound y- (or i- as in Ioreth).[5]

Diphthongs were expressed by tehtar. Two amatixi (maybe a remnant of the earlier Tyelpetéma), used for -i in the diphthongs ai, ei and ui[6]. Au (spelled -aw at the end of words) is not attested, but perhaps similarly the w-tehta was used (caun, a]é6
).

ae, although a diphthong, is attested written with the separate letters for a and e, and maybe the same applies to the diphthong oe too.

A well-known tehta that indicates long consonants, the under-tilde, was not used. Long m and n were written with Malta and Númen (i.e., doubling of the lúva), and other long consonants were written with two letters (mellon, yljjh6
).

A tehta was used to indicate long vowels, an acute accent called Andaith (á, ]R
)

A Elbereth Gilthoniel was an aerlinn of the Elves of Imladris. Imladris was inhabited by the refugees of Eregion[7] and is one of the places that might have kept the Mode of Beleriand, even in the 3rd Age.

] ljwl7l3 s~j3h6lj

A Elbereth Gilthoniel

8~j~r7l6 ql5] 6~V7lj

silivren penna míriel

h yl6lj ]sj]7 ljl6]3

o menel aglar elenath!

6]d]l7l2 q]j]62~V7lj

Na-chaered palan-díriel

h s]j]47lt~6 l5h7]3

o galadhremmin ennorath,

e]6.Öjh8 jl j5]3h6

Fanuilos, le linnathon

6lr ]l]7 8~V 6lr ]l]7h6

nef aear, sí nef aearon!

Notes

↑ This letter is not attested in any Sindarin text, neither is the sound it theoretically represented, but we can theorize that this sound existed in very early Sindarin (g disappeared in the middle of words via this sound. Cf. early Quenya ȝ). If the Noldor came to Middle-earth when this sound still existed, it is likely that they represented it by this tengwa, before finally disappearing. See also Gasdil.

↑ 2.02.1 Because of their doubled lúvar, those tengwar were considered as lenghtened forms of Vala and Órë, which had the values of m and n, respectively.

↑ Maybe for prevocalic and final w (e.g., tawarwaith, tiw). If that's the case, it shouldn't be used for the diphthong au (finally spelled -aw).

↑ 4.04.1 These letters were used for the initial soft sounds rh and lh (spelled hr and hl in Quenya).

↑ This letter (previously Ázë) became ss for Quenya writing. But it is not known which took it from which. There are two equal probabilities

↑ Sindarin didn't possess palatalized sounds. hy had became h. Hyarmen was employed for this soft sound, not to be confused with ch.

↑ This mysterious letter, said to have the value hw, had never appeared in any of the Sindarin texts. It is unknown why was it employed instead of Hwesta and when. It was probably used much later by the Mannish modes, where Hwesta was used for the sound ch or chw.

↑ The lenited g sound was indicated by a sign called Gasdil, which is an elaboration of the letter Halla. It is used like the apostrophe, to indicate a missing g: galadh, "tree", but i ‘aladh, "the tree"

↑ This original letter is similar to Silme Nuquerna however it is a different shape. The sound it represents is the vowel y (not to be confused with the consonantal Quenya y in yulma)